feat: strip ignored files from --changed-only

[llimllib]

--changed-files aka -c was listing .git and ignored files, which it shouldn't have been

Before

$ git ls -c
On branch more-changed

 *      .git
 I      git-ls
MM ++-- main.go      079d7ed 2026-04-09 Bill Mill chore: bump to 5.9.0
MM ++-- main_test.go cad5a9c 2026-04-09 Bill Mill fix: handle detached HEAD and rebase states (#45)
 I      mise.toml

After

$ ./git-ls -c
On branch more-changed

MM ++-- main.go      079d7ed 2026-04-09 Bill Mill chore: bump to 5.9.0
MM ++-- main_test.go cad5a9c 2026-04-09 Bill Mill fix: handle detached HEAD and rebase states (#45)